1. Field of the Invention
The present invention relates to a hydraulic control device for a hydraulic working machine.
2. Description of the Related Art
When a large load acts on an actuator such as a hydraulic cylinder in the hydraulic circuit of a working machine, the internal pressure of the hydraulic circuit is raised. This circuit has a relief valve in order to protect hydraulic equipment within the circuit. When a circuit pressure exceeds a relief pressure, working oil is released to a tank through the relief valve. Accordingly, the pressure is prevented from becoming excessive to prevent the breakage of the hydraulic equipment.
However, most of the working oil supplied from a pump is directly released to the tank without being supplied to the actuator. Therefore, most of the pump power is consumed as the loss of the relief valve, causing a reduction in energy efficiency.
Therefore, a cut-off control for reducing the discharge flow rate of the hydraulic pump is performed in the operation of the relieve valve.
Concretely, as a means for reducing the relief flow rate and relief loss, a throttle means is provided on the downstream side of the relief valve to reduce the pump flow rate when the upstream pressure of the throttle means is increased (e.g. Japanese Patent Application Laid-Open No. 10-246204) or to reduce the pump flow rate when the temperature of the relief valve is raised (e.g., Japanese Patent Application Laid-Open No. 2002-038536).
In the former, however, since the throttle means is provided on the downstream of the relief valve, a pressure loss is generated by the throttle means, and the energy efficiency of the system cannot be improved sufficiently. Further, when a sudden lever operation is performed, a sudden change in pressure is caused because the pump flow rate is suddenly reduced with the sudden increase in the relief flow rate, and hunching is caused according to this.
In the latter, since the temperature of the relief valve is raised with time lag (time delay) from the generation of the relief flow rate, time lag is generated between the generation of the relief flow rate and the cutoff of the pump flow rate, and a sufficient relief loss reducing effect cannot be obtained. Further, since residual heat is detected even after the relief flow rate becomes 0, the pump flow rate is cut off, and a required driving force cannot be immediately obtained.